Issues affecting the global environment are drawing the attention in various paper-related fields of late, such as “global warming” and “release of dioxins and other harmful substances into the environment as a result of waste incineration.” As a way to address these global environment issues through recycling of paper materials, utilization of recycled paper has become popular. However, high-quality recycled pulp itself is becoming less accessible due to widening applications of recycled pulp, higher blending ratios of recycled pulp in paper products, and growing exports of recycled pulp to China, etc. In light of these circumstances, reduction of pulp consumption is desired, as it will not only contribute to the preservation of our global environment, but it will also permit effective utilization of forest resources.
Key applications of paper include those involving fixation of some sort of information onto paper surface. In these applications, paper surface plays the most important role. Specifically, it is desired that a large paper surface area be provided with the minimum amount of pulp, and when printability, transportability, shipment and other handling properties of paper are considered, paper whose weight per unit area is small without changing its thickness, or specifically low-density paper or bulky paper, is preferred in these applications. Here, “fixation of information” refers to fixing text, image or other information onto paper surface by means of printing presses, copiers, and various printers. The paper media onto which information is fixed include, for example, wrapping paper, book paper, newsprinting paper, and various types of so-called information recording paper (electrophotographic transfer paper, inkjet recording paper, thermo-sensitive recording paper, pressure-sensitive recording paper, etc.). From the viewpoint of storage stability of paper on which information has been fixed, paper obtained by a neutral papermaking process is more desirable than paper obtained by an acid papermaking process.
Turning to printing paper such as book paper, which is one area of paper application, the types of popular books are quickly shifting from those of serious contents to more casual comic books and paperbacks, as today's generations are fast abandoning books in favor of other information media. In general, these comic books and paperbacks must be easy to carry and lightweight. For this reason, one key quality requirement demanded by publishers and other users of printing paper is lightness of paper. Reducing the weight of paper means reducing its density while maintaining the thickness, or more specifically, making the paper bulkier.
As conventional technologies to increase paper bulk, one method is to use a bulky pulp composition obtained by reacting a mixture of pulp and hydrophilic fiber with a bridging agent (refer to Patent Literature 1). Another method is to mix cellulose pulp, composite polyester fibers of specific type, and thermo-fusing binder, to produce a bulky pulp sheet (refer to Patent Literature 2). However, use of bridging pulp, synthetic fiber, etc., will make the resulting paper non-recyclable. Yet another method that has been proposed is to produce a bulky book binding paper by using pulp made from a specific tree type (refer to Patent Literature 3). However, it is difficult to produce pulp from a single type of trees in today's environment where the sentiment against logging is becoming increasingly strong.
On the other hand, a number of methods have been examined to produce a bulky paper using internal fillers. One such technology is to produce a bulky neutral paper by filling hollow, spherical vaterite calcium carbonate (refer to Patent Literature 4). However, this is a special filler and its utilization in practical applications is difficult when productivity is considered. Another method that has been disclosed is to produce a bulky printing paper by using amorphous silica or silicate with a bulk specific gravity of 0.3 g/cm3 or less (refer to Patent Literature 5). If a substance of such low specific gravity is used as a filler, however, paper bulk will increase but paper stiffness and sizing property will decrease. Also, such filler results in lower opacity compared to calcium carbonate, etc., and its use in printing paper prevents achievement of good printability.
If the aforementioned printing paper is to be used for offset printing, one essential requirement from the viewpoint of ensuring printability is to add to the paper a certain characteristic needed to suppress water permeation into the paper (sizing property). Paper using white carbon is unable to maintain good printability because its sizing property is significantly lower. In general, fillers of large BET specific surface areas are known to have the tendency to reduce the effects of sizing agents. White carbon is constituted by primary particles agglutinating with one another to exhibit higher bulk and oil absorbency compared to other fillers. On the other hand, however, white carbon is also associated with a significantly larger specific surface area than other fillers. This is considered one reason why use of white carbon as an internal filler reduces the effects of sizing agents. As a result, the obtained paper exhibits poor sizing property.
Turning to newsprinting paper, which is another area of paper application, newspaper printing technology is fast advancing in the areas of offset printing, color printing and high-speed volume printing. Accordingly, the printing medium, or newsprinting paper in this case, is being required to offer better color printability and printing efficiency. In particular, the need to prevent strike-though, which is a phenomenon where text and image printed on one side of paper shows through on the other side, is becoming stronger each year to ensure better opacity after printing. Also, newsprinting paper is becoming increasingly lighter to improve printing efficiency and also to reduce newspaper shipping costs. Improvement of strike-through is one essential requirement that must be addressed in the effort to reduce the weight of newsprinting paper.
Achieving higher paper opacity and oil absorbency is known as the most effective way to reduce strike-through of newsprinting paper. One way to effectively increase opacity is to add titanium dioxide having a high specific scattering coefficient. However, titanium dioxide is expensive and it is not economical to add it to paper in large quantities. Another effective way to suppress strike-through of acid newsprinting paper is to add fillers of high oil absorbency, and white carbon has been used widely for this purpose.
One new technology that has emerged with respect to newsprinting paper is a neutral process of making newsprinting paper. Specifically, a neutral newsprinting paper containing 5 to 15 percent by weight of calcium carbonate as a filler, as well as its production method, are disclosed with the aim of providing a neutral newsprinting paper offering strength, opacity, resin retention, and wear resistance in offset printing, of levels equivalent to or better than those offered by acid newsprinting paper (refer to Patent Literature 6).
Also, technologies are known that relate to the blending of fillers in neutral newsprinting paper. For example, a method of producing paper (including newsprinting paper) is presented, which uses as a filler a mixture of hydrated silicate and precipitated calcium carbonate or ground calcium carbonate, exhibiting an oil absorbency of 250 to 350 ml per 100 g and having specific particle characteristics including a total pore volume of 4.0 to 6.0 cc/g, average pore radius of 200 to 400 angstroms, and average particle size of 3.0 to 15 μm as measured by the laser method (refer to Patent Literature 7). In addition, newsprinting paper is disclosed in which a filler, mainly made of white carbon and calcium carbonate, is mixed with SiO2 and CaO at a ratio of 9:1 to 5:5 based on ash atomic absorption spectrophotometry at 550° C. as specified in JIS P 8128 (refer to Patent Literature 8). However, the aforementioned technologies do not provide sufficient opacity or strike-through suppression effect. Separately, a neutral newsprinting paper containing titanium dioxide-calcium carbonate composite and having a grammage of 45 g/m2 or less is disclosed (refer to Patent Literature 9). However, this paper uses expensive titanium dioxide, which is not desirable.
Turning to information recording paper such as electrophotographic transfer paper, pulp is used to increase paper bulk, i.e., to reduce paper density. Among the types of pulp that can be used for this purpose, ground pulp produced by grinding wood using a grinder, refiner mechanical pulp produced by fine-grinding wood using a refiner, and thermo-mechanical pulp and other types of mechanical pulp, provide rigid fibers and are more advantageous to density reduction compared to chemical pulp produced by using chemicals to extract lignin, a type of reinforcement material, from wood fibers. However, ground pulp and refiner pulp receive strong mechanical friction and shear force and thus pulp fibers become finer and shorter. As a result, paper strength decreases.
For the aforementioned reason, traditionally mechanical pulp is made from coniferous trees with long fibers. However, coniferous trees contain a lot of lignin and thus are unable to produce pulp offering high levels of brightness. In particular, conventional mechanical pulp is not suitable for electrophotographic transfer paper, where high brightness is often required to provide better color reproducibility and higher product value in line with the growing need for color printing. Among the various types of mechanical pulp, thermo-mechanical pulp does not present the problems associated with fine fibers, and is able to maintain sheet stiffness. However, paper made from thermo-mechanical pulp offers poor smoothness and insufficient image reproducibility. For example, a method is disclosed wherein thermo-mechanical pulp (TMP) or other high-yield pulp is used to reduce grammage while maintaining opacity and stiffness (refer to Patent Literature 10). However, paper produced by such method provides poor properties such as brightness, and does not offer sufficient color reproducibly to meet the requirement for color printing.
Various ways are also examined to increase paper bulk through the use of inorganic fillers, etc., offering lower specific gravity than pulp. For example, use of the filler disclosed in Patent Literature 5, made of amorphous silica or silicate with a bulk specific gravity of 0.3 g/cm3 or less, does increase bulk by keeping paper density low. If this filler is used for electrophotographic transfer paper, however, paper stiffness drops, which in turn results in reduced suitability for copying such as poor run and low work efficiency.
[Patent Literature 1] Specification of Japanese Patent No. 2903256
[Patent Literature 2] Specification of Japanese Patent No. 2591685
[Patent Literature 3] Specification of Japanese Patent No. 1755152
[Patent Literature 4] Specification of Japanese Patent No. 3306860
[Patent Literature 5] Specification of Japanese Patent No. 2889159
[Patent Literature 6] Specification of Japanese Patent No. 2960002
[Patent Literature 7] Japanese Patent Laid-open No. 2002-201590
[Patent Literature 8] Japanese Patent Laid-open No. 2002-201592
[Patent Literature 9] Japanese Patent Laid-open No. 2002-38395